In the commercial preparation of phosphorus, a mixture of calcium phosphate usually as phosphate shale; carbon, such as coke; and sand (silica, SiO.sub.2) flux is charged into an electric furnace and the contents heated to melting temperatures, generally about 3000.degree. F. (about 1650.degree. C.). The phosphate is reduced by the carbon reductant to elemental phosphorus, which is evolved as a vapor from the molten reaction mass. The vapor is conveyed to a cooling zone and condensed. Molten slag is drawn off through an outlet at the bottom of the furnace. Also drawn off through a lower tap hole is the dense, metallic like ferrophosphorous, an alloy of phosphorus, iron and other heavy metals.
Although the electric furnace process of smelting phosphate rock to obtain phosphorus has been carried out for a number of years, it has several disadvantages. Chief among these are the high consumption of electric power and the generation of slag waste products that require safe handling and disposal. In some installations, specially prepared briquettes of hardened carbon reductant and phosphate shale are used in formulating the furnace feed materials. Such items together with the expensive electrode equipped furnaces contribute further to plant and operating costs.
Depending on the source of the phosphate ore and the refining conditions, elemental phosphorous produced by the electric furnace process typically contains parts per million quantities of one or more heavy metals, such as zinc, chromium, arsenic, and antimony. These metals can be carried over when the phosphorous is converted to phosphoric acid, making the acid unacceptable in certain high value applications, such as in food and in semiconductor applications. Removal of these metals from the phosphorous before it is converted to phosphoric acid typically requires expensive purification processes, which may not be economical if high levels of these metals are present.
Phosphoric acid can be produced directly by addition of an acid, such as sulfuric acid, to phosphate rock. Although the composition of the resulting acid, known as "wet phosphoric acid", varies with the composition of the phosphate rock, the acid is typically impure and not suitable for food grade and for semiconductor applications. The acid typically contains high levels of fluoride and other materials, such as iron and aluminum, that must be removed by extensive purification processes before the acid is suitable for these high value applications.
Thus, a need exists for a method of preparing phosphorous that does not produce large amounts of slag and which produces phosphorous that does not contain large amounts of heavy metal contaminants as well as for a method for preparing phosphoric acid suitable for food grade and other high value applications that does not require extensive purification.